What is known in the design of nuclear power plants as a "maximum design basis accident" implies a hypothetical accident involving loss of coolant which may occur as a result of an instantaneous rupture of the largest pipeline followed by an unimpeded outflow of the coolant from both sides of the break. In the case of an accident involving loss of coolant, radioactive products escape together with the coolant into the reactor room wherein a pressure rise occurs as a result of vaporization caused by ebullition of the coolant. This give rise to the possibility of release of radioactive products to the environment. To preclude contamination of the environment, nuclear power plants include hermetically sealed containments retaining the radioactive products released in the course of an accident.
Such hermetically sealed containments are designed for a maximum pressure of the vapor produced in the case of an accident at the expense of the entire released energy. Since in the event of an accident involving loss of coolant considerable quantities of vapor are produced, large-volume containments are required which are strong enough to withstand the action of a high-temperature and high-pressure vapor-air mixture. This requirement renders such safety systems substantially costly.
To reduce the costs of safety systems, designers try to minimize the pressure within the containment. This can be attained either by supplying coolant to condense the vapor or by dividing the containment into two chambers.
One of these chambers accomodates the nuclear reactor and its primary cooling circuit, while the other is intended to contain the air forced out of the first chamber in the case of an unusually high pressure rise as a result of vaporization caused by ebullition of the leaking coolant. Normally, passive condensers are arranged between said chambers.
In the case of an accident involving loss of coolant, the vapor produced as a result of boiling of the coolant mixes with the air with which the first chamber was filled prior to the accident, and the pressure in the first chamber rises to become higher than in the second chamber. The resulting pressure difference causes the vapor-air mixture to flow into a condenser in which the vapor is condensed, while the air enters the second chamber wherein the pressure starts to increase. Used as the passive vapor condensers are, for example, ice condensers or containers filled with water wherethrough the vapor-air mixture is bubbled. In the course of bubbling of the vapor-air mixture, the vapor is condensed, while the air passes through the water to enter the second chamber. To provide for an adequate condensing capacity, large amounts of water should be used. The use of containers with large amounts of water is apt to cause hydraulic shocks as the vapor-air mixture is being bubbled through the water, whereby stricter requirements have to be imposed on the mechanical strength of such containers and the costs involved in their construction become unduly high. Despite the employment of means for reducing the pressure in the above-described safety devices, excess pressure persists for a long period of time, and since the containment cannot be absolutely hermetic, it is practically impossible to completely rule out the possibility of escape of radioactive products to the environment. In order to keep the level of radioactive contamination below the tolerable limit, the degree of hermeticity of the containment should be extremely high, which is another factor adding to the total cost of the safety arrangement.
Also known in the art are systems for limiting the consequences of an accident, comprising two rooms, one of which accommodates a nuclear reactor with its cooling circuit, and in the other room, communicating with the first room through a valve, a pressure below atmospheric is permanently maintained by exhausting air therefrom. In the case of an accident involving loss of coolant, a vapor-air mixture enters the second room which accommodates a vapor condenser. The negativepressure in the second room prior to the accident and condensation of the vapor as the vapor-air mixture comes in during the accident result in the pressure in the second room remaining below atmospheric. The initial exhaustion in the second room being sufficient, the pressure in the first room can be brought to a subatmospheric level, too. However, such a system involves unduly high expenditures to build said second room and maintain negative pressure throughout the operating period.
Additionally, all the nuclear plant equipment, including the reactor, cooling circuit, fuel recharging system and the like, is enclosed in a containment. In the course of normal operation of the reactor, minor leakage of the coolant may occur with radioactive products of fission escaping therewith into the space confined within the containment, whereby the atmosphere in the containment may go below the radiation safety standards. Therefore, under normal operating conditions, access to the nuclear plant equipment enclosed in a hermetically sealed containment is limited.